THE STRUCTURES OF THE ETA-CARBIDES NI6MO6C, CO6MO6C, AND CO6MO6C2

The structures of the three eta -carbides Ni//6Mo//6C, Co//6Mo//6C, and Co//6Mo//6C//2, prepared by a new synthetic route, have been determined by Rietveld analyses of monochromatic neutron powder diffraction data. These studies demonstrate that the compound precursor synthesis route used yields high-quality, homogeneous materials and provides precise structural details for the eta -12 and eta -6 carbide phases. The differences between these two structures are discussed. The complete absence of solid solubility between the eta -12 and eta -6 phases is considered to reflect the limited separation of only 2. 4 angstrom between the 8a and 16c carbon atom sites.